Many of the conventional and combined cycle (CCGT) power plants, which have been in operation for a number of years, are presently being subjected to frequent start-stop cycles for longer periods of time. The reasons for this change in operating mode (from base load to cyclic operation) are related to economic factors, environmental drivers, market regulations, privatization of the power industry and the increasing use of non-dispatchable power sources. Whatever the reasons for change in the operation mode, these plants need to meet market demands and those that cannot do this can be penalized. Previous studies carried out by ETD have shown that the older plants, when subjected to cyclic operation, exhibit new damage types at a relatively short time after the start of the cyclic mode operation compared with the newer plants. Thus, it is clear that plants which have undergone changes in operating mode from base load to cycling need appropriate guidance for the improvement of their existing maintenance programs in order to ensure significant savings, higher operational flexibility, faster response and improved profitability.
Analysis of the survey carried out as a part of this project revealed that the changing of operation mode from base load to cycling has great impact on plant performance as well as maintenance programs. In this work, various performance metrics, such as equivalent forced outage factor (EFOF), equivalent planned outage factor (EPOF), availability, reliability, etc., were analyzed to determine the impact of cycling. The performance analysis identified three distinct areas for the entire life cycle of a power plant operating either in base load or cycling regime, which form the classic ‘bathtub’ shape curve (typical to most plant equipment). Analysis of Reliability (R) and Availability (A) revealed lower values during the earlier plant life and higher values during the later part of the plant life. A case study presented in this report for a group of conventional and CCGT units, which were originally designed for base load and after a certain period of operation were converted to cycling, also supports these analysis results.
The exact nature of the cyclic operation does have a significant effect on the maintenance plans (i.e. the changes that need to be made from the standard base load maintenance plan). In general, a maintenance program for a power plant includes the maintenance schedule, selection of inspection type and selection of inspection scope (selection of locations/areas to be inspected). The analysis revealed that cycling units require more maintenance due to additional damages/issues (which are mostly related to fatigue damage) and, therefore, more focused inspections are required. A summary of general guidelines is developed in this report for maintenance of the cycling units. The report also provides maintenance activities including inspection guidelines and recommendations for component based damage inspection for both base load and cycling units.
This report has been divided into different sections to make it easier to follow. References have been provided at the end of each section for the reader to gain further background information, if needed.
Conventional plant, Combined Cycle Power Plant (CCGT), Base load, Cycling, Switching operation, Maintenance program, higher operational flexibility, Performance, Availability, Reliability, Outage interval, Inspection scope, Failure, Cycling issues, Fatigue Damage, Guidelines